After the Senate elections in 2018. Democrats need to win majority.
I wouldn't hope on that. Off years, democrats always have a low turnout at polls.
But they would need to win majority of house as well. The house is what brings the impeachment up. The senate only does the trial and sentencing.
So they would need the house to first impeach, then the senate to kick him out of office.
and not ot mention it's 2/3 pretty sure, not just a simple majority.